Double-stacked ebg structure

ABSTRACT

In a double-stacked electromagnetic bandgap (EBG) structure, a first conductive plane and a second conductive plane are spaced apart in parallel. At least two EBG layers are embedded in parallel between the first conductive plane and the second conductive plane. The at least two EBG layers have different stopband characteristics. A plurality of vias connect the at least two EBG layers respectively to one of the first and second conductive planes. At least the vias connecting one of the EBG layers pass through via holes in cells of another EBG layer.

FIELD OF THE INVENTION

The present invention relates broadly to an electromagnetic bandgap(EBG) structure, to an electronic device, and to a method forsuppressing electromagnetic noise in an electronic device.

BACKGROUND

In the semiconductor electronic devices that process digital signals andanalog signals at the same time, semiconductor devices processing thedigital signals and semiconductor devices processing the analog signalsare separately fabricated and then are assembled together. As thesemiconductor electronic devices are scaled down, techniques forprocessing the digital signals and the analog signals within a singlesemiconductor device have been developed.

Bluetooth modules or RFID modules are representative semiconductordevices having both digital signal processing circuits and analog signalprocessing circuits. These modules include digital circuits (e.g.,memory, arithmetic unit, etc.) and RF analog circuits (e.g., RF amp,PLL, antenna, etc.) within a single semiconductor package. These modulesare called a mixed signal system. In a single semiconductor package, aplurality of semiconductor devices and a plurality of passive circuitsmanufactured by various processes are integrated into a single system.Thus, the semiconductor package is typically known as asystem-in-package (SiP).

In the system-in-package processing the mixed signals, a digital circuitpart and an analog circuit part may share a common power plane and aground plane parallel to each other, and may use them separately. In anycase, the two circuit parts are coupled directly or indirectly throughvarious electromagnetic mechanisms. An issue is a wideband switchingnoise that is generated by a switching operation and clock signal of thedigital circuit part and is inherently propagated to the analog circuitpart. The power plane/ground plane may be considered as a kind of aparallel plate waveguide. A plurality of vias formed in the powerplane/ground plane operates as an antenna receiving the switching nose.Because the switching noise has a wideband, it overlaps an analog signalband at which the analog circuit part operates. In addition, because theanalog circuit is very sensitive to the switching noise, it is veryimportant to suppress the switching noise.

Various approaches to reduce the switching noise when the parallel platepower plane/ground plane have been developed. Examples of the approachesinclude a method for suppressing resonance generated in a cavity betweentwo plates, for example, a method for attenuating RF signals usingabsorbent or loss component, and a method for dividing a power plane.However, these methods are effective only to an electromagnetic wavehaving a band of a few hundreds MHz and a limited directionality andwithin a restricted region.

An electromagnetic bandgap (EBG) structure has been developed tosuppress a surface current generated in an RF analog device. The EBGstructure is inserted between the power plane and the ground plane andoperates as an RF bandstop filter. The EBG structure is very effectiveto an electromagnetic wave of GHz having a planar omnidirectionalcharacteristic and extends along the SiP and can be implemented at a lowcost.

FIG. 1 is a perspective view of a conventional EBG structure. Referringto FIG. 1, the conventional EBG structure 10 includes a power plane 11,a ground plane 12, and an EBG layer 13. The power plane 11 and theground plane 12 are arranged parallel to each other, and the EBG layer13 is embedded between the two planes 11 and 12. The EBG layer 13 isconnected to one of the two planes 11 and 12 through vias 14. In thecase of FIG. 1, the EBG layer 13 is connected to the ground plane 12.The EBG layer 13 is divided into cells that are repetitively arranged atconstant periods. The via 14 connects the EBG layer 13 to one of the twoplanes 11 and 12 at each cell. A dielectric having a predeterminedpermittivity is filled between the EBG layer 13 and the two planes 11and 12. Because a low-temperature co-fired ceramic (LTCC) has afrequency stable permittivity property and a low loss, it is widely usedas the dielectric.

The ground plane 12 and the EBG layer 13 have a self-inductance that isdetermined depending on their physical shapes. The power plane 11 andthe EBG layer 13 have a predetermined capacitance that is determineddepending on the gap between the cells, the permittivity of the fillmaterial, and the size of the cells. A stopband center frequency of theEBG structure 10 changes depending on the self-inductance and thecapacitance. Specifically, it is known that the stopband centerfrequency is proportional to √{square root over (L/C)}. Therefore, theEBG structure 10 can set the desired suppression band as the stopband bydetermining the L/C ratio. As the capacitance increases, the stopbandwidth increases.

In this manner, by adjusting the gap, permittivity, and size of thecells of the EBG layer 13, the bandstop filter having stopbands ofdifferent center frequencies and different bandwidths can be implementedusing the EBG structure 10. For example, when the cell size is small,the capacitance becomes small and the frequency of the stopband becomeshigh. In this case, however, the stopband width becomes narrow. When thestopband width has to be large for a small size of the package or board,the conventional EBG structure 10 cannot properly meet such arequirement.

Double-stacked EBG structures have also been proposed, to provide abroader effective stopband bandwidths. In such double-stacked EBGstructures, the individual EBG layers can be designed to achievedifferent center frequencies and stopbands. More particular, twodifferent cell sizes can be used for the two EBG layers, andalternatively or additionally different permittivity dielectric layerscan be used for the respective EBG layers to achieve different centerfrequencies and stopbands. However, the proposed structures have thedisadvantage of limiting the size variations of the cells due toaccommodating respective vias for connection of the cells to the powerthe ground planes in gaps between the cells. As a result, the proposeddesigns are limited by low start frequencies for the effective stopbandbandwidths. Furthermore, design variation based on using differentpermittivity dielectric layers has the disadvantage of having tointegrate multiple dielectrics with different mechanical and processingproperties.

A need therefore exists to provide an alternative double-stacked EBGstructure that seeks to address at least one of the above problems.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention there isprovided a double-stacked electromagnetic bandgap (EBG) structurecomprising a first conductive plane and a second conductive plane spacedapart in parallel; at least two EBG layers embedded in parallel betweenthe first conductive plane and the second conductive plane, the at leasttwo EBG layers having different stopband characteristics; and aplurality of vias for connecting the at least two EBG layersrespectively to one of the first and second conductive planes; whereinat least the vias connecting one of the EBG layers pass through viaholes in cells of another EBG layer.

The at least two EBG layers may be formed such that the stopbands arecontiguous.

The at least two EBG layers may have different cell sizes.

The first conductive plane may be a power plane and the secondconductive plane may be a ground plane.

The cell size of the EBG layer connected to the power plane may besmaller than that of the EBG layer connected to the ground plane.

The cells may have a substantially rectangular shape.

The double-stacked EBG structure may further comprise a non-conductivelayer or a dielectric layer formed between the conductive planes and theEBG layers.

The dielectric layer may be a low-temperature co-fired ceramic (LTCC).

The dielectric layers between the conductive layers and the respectiveEBG layers may have a uniform thickness.

A ratio of a cell size in one EBG layer to a cell size in another EBGlayer may be equal to or greater than 2:1.

Each cell in one EBG layer may overlap an integer number of cells inanother EBG layer.

In accordance with a second aspect of the present invention there isprovided an electronic device comprising an electronic circuit having apredetermined function; and a board for suppressing propagation ofswitching noise, the board including a first conductive plane and asecond conductive plane spaced apart in parallel; at least two EBGlayers embedded in parallel between the first conductive plane and thesecond conductive ground plane, the at least two EBG layers havingdifferent stopband characteristics; and a plurality of vias forconnecting the at least two EBG layers respectively to one of the firstand second conducive planes; wherein at least the vias connecting one ofthe EBG layers pass through via holes in cells of another EBG layer.

The electronic circuit may be a mixed signal semiconductor circuit toprocess both a digital signal and an analog signal, and the electronicdevice is a system-in-package.

The at least two EBG layers may be formed such that the stopbands arecontiguous.

The at least two EBG layers may have different cell sizes.

The first conductive plane may be a power plane and the secondconductive plane may be a ground plane.

The cell size of the EBG layer connected to the power plane may besmaller than that of the EBG layer connected to the ground plane.

The cells may have a rectangular shape.

The electronic device may further comprise a non-conductive layer or adielectric layer formed between the conductive planes and the EBGlayers.

The dielectric layer may comprise a low-temperature co-fired ceramic(LTCC) dielectric layer.

The dielectric layers between the conductive layers and the respectiveEBG layers may have a uniform thickness.

A ratio of a cell size in one EBG layer to a cell size in another EBGlayer may be equal to or greater than 2:1.

Each cell in one EBG layer may overlap an integer number of cells inanother EBG layer.

In accordance with a third aspect of the present invention there isprovided a method for suppressing electromagnetic noise in an electronicdevice having a first conductive plane and a second conductive plane,comprising forming at least two EBG layers having different stopbandsbetween the first conductive plane and the second conductive plane inparallel; and connecting the at least two EBG layers respectively to oneof the first and second conductive planes through a plurality of vias;wherein at least the vias connecting one of the EBG layers pass throughvia holes in cells of another EBG layer.

The at least two EBG layers may be formed such that the stopbands arecontiguous.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the present generalinventive concept will become apparent and more readily appreciated fromthe following description of the embodiments, taken in conjunction withthe accompanying drawings of which:

FIG. 1 is a perspective view of a conventional EBG structure;

FIG. 2 is a perspective view of a double-stacked EBG structure accordingto an embodiment of the present invention;

FIG. 3 is a cross-sectional view of the double-stacked EBG structure ofFIG. 2;

FIG. 4 is a cross-sectional view of a system-in-package having adouble-stacked EBG structure according to an embodiment of the presentinvention;

FIGS. 5A, 5B and 5C are graphs illustrating results when a switchingnoise signal is measured in time domain in case where thesystem-in-package has no the EBG structure, in case where thesystem-in-package has the EBG structure of FIG. 1, and in case where thesystem-in-package has the EBG structure of FIG. 2, respectively; and

FIGS. 6A and 6B are graphs illustrating results when a power noisecoupling coefficient between two positions on a power plane is measuredin frequency domain in case where the system-in-package has the EBGstructure of FIG. 1 and in case where the system-in-package has the EBGstructure of FIG. 2, respectively, and in both cases compared to thecase where the system-in-package has no EBG structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is a perspective view of a double-stacked EBG structure accordingto an embodiment of the present invention.

The double-stacked EBG (DS-EBG) structure includes two or more EBGlayers between a power plane and a ground plane spaced apart inparallel. Referring to FIG. 2, the double-stacked EBG structure has twoEBG layers, that is, a first EBG layer 23 and a second EBG layer 24,between the power plane 21 and the ground plane 22.

The first and second EBG layers 23 and 24 are divided into small cells.The cell size of the first EBG layer 23 is different from that of thesecond EBG layer 24. The cells can have various shapes. For example, thecells may be square, hexagon, and so on. In any case, it is preferableto make efficient use of the area.

Generally, the cell size and the gap between the cells have a closerelation to frequency and bandwidth of a desired stopband. Also, thespacing between vias is preferably smaller than the wavelengthcorresponding to the center frequency of the stopband. For this reason,the spacing between the vias is determined depending on the centerfrequency of the stopband. As mentioned in the background section, inexisting double-stacked EBG structures, the spacing between the vias inturn imposes a limitation on the cell size, as a result of accommodatingthe vias in the gaps between the cells. Therefore, to adjust theinductance and the capacitance of the EBG structure, the overall heightof the EBG structure, different dielectric materials, or both, have tobe adjusted. In practice, however, the limitation of the entire size ofthe system or board, as well as manufacturing issues of integratingmultiple dielectric materials with different mechanical and processingproperties, make it difficult to design such existing EBG structures tohave the desired inductance and capacitance.

In contrast, in the EBG structure 20 shown in FIG. 2, for at least oneof the EBG layers 23, 24, the connecting vias pass through via holesformed in the other EBG layer. As a result, advantageously, the cellsize of at least one of the EBG layers 23, 24 is not limited byaccommodating the vias in gaps between the cells. Therefore, the spacingbetween vias, which is determined depending on the center frequency ofthe stopband as mentioned above, does not impose the undesiredlimitation on the cell size of both EBG layers.

The EBG layer 23 is connected to the ground plane 22 through the firstvia 25, and the second EBG layer 24 is connected to the power plane 21through the second via 26. The first via 25 may be connected to theground plane 22 while passing through an empty space where edges of thecells of the second EBG layer 24 are contacted. The second via 26 passesthrough a via hole 27 formed in the first EBG layer 23. It was foundthat the via hole 27 can have a negligible influence on theelectromagnetic characteristic of the first EBG layer 23.

In LTCC technology manufacturing as an example of implementation, viaholes 27 can be formed by punching or drilling in a ceramic tape andforming the pattern for the EBG layer 23 on one surface of the ceramictape around the drill holes by printing or other known techniques.Stencil filling is then used to fill the holes, thereby forming thefilled vias. Multiple tape layers are laminated to form thedouble-stacked EBG structure 20, Similarly, the blind vias, e.g. 25, canbe readily achieved using LTCC.

The first EBG layer 23 has about four times the cell size of the secondEBG layer 24, and the second EBG layer 24 has about four times thefrequency of the stopband center frequency of the first EBG layer 23.The first EBG layer 23 and the power plane 21 form one EBG structure,and the second EBG layer 24 and the ground plane 22 form another EBGstructure. Because the first EBG layer 23 is embedded between the powerplane 21 and the second EBG layer 24, or the second EBG structure 24 isembedded between the ground plane 22 and the first EBG layer 23, thedouble-stacked EBG structure 20 can be implemented at a thickness equalto that of the conventional single-layer EBG structure 10.

The first EBG layer 23 and the second EBG layer 24 have differentpotentials. Additionally, cells in the EBG layer 23 overlap completelywith an integer number of cells in the EBG layer 24, which results in anincreased capacitance coupling between the EBG layers 23, 24, comparedto existing double-stacked EBG structures in which vias connecting thecells are accommodated in the gaps between the cells of the other EBGlayer. This advantageously can generate higher order EBG modes andmultiple transmission zeros, and capacitance and inductance can beadditionally obtained between the first EBG layer 23 and the second EBGlayer 24. As described above, the capacitance and inductance of the EBGstructure are an important factor in determining the center frequencyand frequency band of the stopband. Therefore, the use of thedouble-stacked EBG structure 20 makes it possible to design for thecenter frequency and frequency band of the desired stopband more freely.

The double-stacked EBG structure 20 has two EBG structures that providethe sub stopbands having the center frequency and frequency banddetermined by the physical specification. The entire stopband has ashape formed by the combination of the sub stopbands. If the centerfrequency and the frequency band are determined such that two substopbands are contiguous, the entire stopband can have a very widebandwidth.

FIG. 3 is a sectional view of the double-stacked EBG structure 20.Referring to FIG. 3, the first EBG layer 23 and the second EBG layer 24are arranged between the power plane 21 and the ground plane 22. Thefirst EBG layer 23 is connected to the ground plane 22 through vias 25,and the second EBG layer 24 is connected to the power plane 21 throughvias 26. The first EBG layer 23 and the second EBG layer 24 are dividedinto cells having different sizes. The cell size of the first EBG layer23 is larger than that of the second EBG layer 24. However, in otherembodiments, the cell size of the first EBG layer 23 may be smaller thanthat of the second EBG layer 24. The vias 25 and 26 may be connected tothe planes 21 and 22 at the center portions of the respective cells.Accordingly, the via 26 of the second EBG layer 24 having the small cellsize may be connected to the power plane 21 through the via hole 27 ofthe first EBG layer 23. The dielectric is filled between the layers. Alow-temperature co-fired ceramic (LTCC) is used as the dielectric.

In the double-stacked EBG structure 20, the thickness of each dielectriclayer 30, 32, 34 is advantageously uniform. As mentioned above, inexisting double-stacked EBG structures, due to the limitation on cellsize variation for the EBG layers, typically different dielectricthickness values are required to try and achieve a desired centerfrequency and bandwidth of the stopband. While the use of differentdielectric thickness values can be readily accommodated in systems basedon printed circuit board (PCB) manufacturing processes, such differentdielectric thickness values are typically undesirable for LTCCmanufacturing processes. In contrast, the uniform thickness dielectriclayers 30, 32 and 34 utilized in the double-stacked EBG structure 20 arereadily suitable for standard LTCC manufacturing processes, and canadvantageously achieve an implementation with a thinner total substratethickness compared to existing double-stacked EBG structures.

FIG. 4 is a sectional view of a SiP having a double-stacked EBGstructure according to an embodiment of the present invention.

Referring to FIG. 4, the system-in-package 40 includes a semiconductorchip 41 and a package board 42. The semiconductor chip 41 is a mixedsignal system and includes a digital circuit part and an analog circuitpart at an upper portion. The package board 42 is provided under thesemiconductor chip 41. The package board 42 has a double-stacked EBGstructure in which a power plane 44 and a ground plane 43 are embeddedand at least two EBG layers 45 and 46 are disposed between the powerplane 44 and the ground plane 43.

FIGS. 5A, 5B and 5C are graphs illustrating results when a switchingnoise signal is measured in the time domain in case where the SiP has noEBG structure, in case where the SiP has the EBG structure of FIG. 1,and in case where the SiP has the EBG structure of FIG. 2, respectively.

The test was performed using a test apparatus that has a 20 mm×20 mm×10μm power plane and a 20 mm×20 mm×10 μm ground plane formed of goldconductor and low-loss LTCC layers having a thickness of 0.1 mm. Thecell size of the EBG layer of FIG. 1 was 3.8 mm×3.8 mm, the cell gap was0.2 mm, a distance from the power distribution surface was 0.1 mm, and adistance from the ground plane was 0.2 mm. In the EBG structure of FIG.2, the cell size of the first EBG layer was 3.8 mm×3.8 mm, the cell gapwas 0.2 mm, and a distance from the power distribution surface was 0.1mm. The cell size of the second EBG layer was 1.8 mm×1.8 mm, the cellgap was 0.2 mm, and a distance from the ground plane was 0.1 mm. The gapbetween the two EBG layers was 0.1 mm.

In FIGS. 5A, 5B and 5C, a vertical axis represents a measured switchingnoise (mV) and a horizontal axis represents time (ns). Referring to FIG.5A, when the EBG structure was not employed, noise was 370 mV. Referringto FIG. 5B, when the conventional EBG structure was employed, noise was80 mV. Referring to FIG. 5C, when the EBG structure of the presentinvention was employed, noise was 12 mV. That is, it can be seen thatnoise was remarkably reduced.

FIGS. 6A and 6B are graphs illustrating results when a power noisecoupling coefficient S21 between two positions 1 and 2 on a power planeis measured in frequency domain in case where the SiP has the EBGstructure of FIG. 1 and in case where the SiP has the EBG structure ofFIG. 2, respectively, and in both cases compared to the case where theSiP has no EBG structure.

The test was performed using the test apparatus described in FIG. 5. InFIGS. 6A and 6B, a dotted line represents the measured power noisecoupling coefficient when the EBG structure is not employed. Thestopband means a band having the measured noise coupling coefficientlower than −30 dB.

Referring to FIG. 6A, when the conventional EBG structure was employed,the stopbands appeared at 3.5 GHz-6.5 GHz (about 3 GHz), 6.6 GHz-12.5GHz (about 6 GHz), and 17.5 GHz-19.5 GHz (about 2 GHz). Referring toFIG. 6B, when the EBG structure of the present invention was employed,the stopbands appear at 3 GHz-16 GHz and 17.5 GHz-21 GHz. The case ofFIG. 6B has a bandwidth of 13 GHz and 3. GHz, which is wider than thecase of FIG. 6A. This illustrates that in the described double-stackedEBG structure, the cell dimensional parameters are designed to achievean overlap of individual EBG stopbands to achieve an increase instopband bandwidth. Since vias for at least one of the EBG layers passthrough via holes in the other EBG layer, ratios of cell sizes betweenthe two EBG layers can e.g. be about 2:1, and can be extended tomultiple of integers 3:1, etc.

In the described double-stacked EBG structure of the present invention,the cells having different areas are arranged in different layers, andthe cells of the different layers are connected to different planes. Thedouble-stacked EBG structure of the present invention can provide thestopband having the wider frequency band compared with the conventionalEBG structure.

The described system-in-package and the printed circuit board having thedescribed double-stacked EBG structure can be implemented formixed-signal (analog and digital) applications.

The described system-in-package and the printed circuit board having thedescribed double-stacked EBG structure can suppress the propagation ofthe switching noise by using the stopband having the wide frequencyband.

It will be appreciated by a person skilled in the art that numerousvariations and/or modifications may be made to the present invention asshown in the specific embodiments without departing from the spirit orscope of the invention as broadly described. The present embodimentsare, therefore, to be considered in all respects to be illustrative andnot restrictive.

For example, although the LTCC-based system-in-package has beendescribed, the present invention can also be applied to a printedcircuit board.

1. A double-stacked electromagnetic bandgap (EBG) structure comprising:a first conductive plane and a second conductive plane spaced apart inparallel; at least two EBG layers embedded in parallel between the firstconductive plane and the second conductive plane, the at least two EBGlayers having different stopband characteristics; and a plurality ofvias for connecting the at least two EBG layers respectively to one ofthe first and second conductive planes; wherein at least the viasconnecting one of the EBG layers pass through via holes in cells ofanother EBG layer.
 2. The double-stacked EBG structure according toclaim 1, wherein the at least two EBG layers are formed such that thestopbands are contiguous.
 3. The double-stacked EBG structure accordingto claim 1, wherein the at least two EBG layers have different cellsizes.
 4. The double-stacked EBG structure according to claim 1, whereinthe first conductive plane is a power plane and the second conductiveplane is a ground plane.
 5. The double-stacked EBG structure accordingto claim 4, wherein the cell size of the EBG layer connected to thepower plane is smaller than that of the EBG layer connected to theground plane.
 6. The double-stacked EBG structure according to claim 1,wherein the cells have a substantially rectangular shape.
 7. Thedouble-stacked EBG structure according to claim 1, further comprising: anon-conductive layer or a dielectric layer formed between the conductiveplanes and the EBG layers.
 8. The double-stacked EBG structure accordingto claim 7, wherein the dielectric layer is a low-temperature co-firedceramic (LTCC).
 9. The double-stacked EBG structure according to claim8, wherein the dielectric layers between the conductive layers and therespective EBG layers have a uniform thickness.
 10. The double-stackedEBG structure according to claim 1, wherein a ratio of a cell size inone EBG layer to a cell size in another EBG layer is equal to or greaterthan 2:1.
 11. The double-stacked EBG structure according to claim 1,wherein each cell in one EBG layer overlaps an integer number of cellsin another EBG layer.
 12. An electronic device comprising: an electroniccircuit having a predetermined function; and a board for suppressingpropagation of switching noise, the board including: a first conductiveplane and a second conductive plane spaced apart in parallel; at leasttwo EBG layers embedded in parallel between the first conductive planeand the second conductive ground plane, the at least two EBG layershaving different stopband characteristics; and a plurality of vias forconnecting the at least two EBG layers respectively to one of the firstand second conducive planes; wherein at least the vias connecting one ofthe EBG layers pass through via holes in cells of another EBG layer. 13.The electronic device according to claim 12, wherein the electroniccircuit is a mixed signal semiconductor circuit to process both adigital signal and an analog signal, and the electron device is asystem-in-package.
 14. The electronic device according to claim 12,wherein the at least two EBG layers are formed such that the stopbandsare contiguous.
 15. The electronic device according to claim 12, whereinthe at least two EBG layers have different cell sizes.
 16. Theelectronic device according to claim 12, wherein the first conductiveplane is a power plane and the second conductive plane is a groundplane.
 17. The electronic device according to claim 16, wherein the cellsize of the EBG layer connected to the power plane is smaller than thatof the EBG layer connected to the ground plane.
 18. The electronicdevice according to claim 12, wherein the cells have a rectangularshape.
 19. The electronic device according to claim 12, furthercomprising: a non-conductive layer or a dielectric layer formed betweenthe conductive planes and the EBG layers.
 20. The electronic deviceaccording to claim 10, wherein the dielectric layer comprises alow-temperature co-fired ceramic (LTCC) dielectric layer.
 21. Theelectronic device according to claim 20, wherein the dielectric layersbetween the conductive layers and the respective EBG layers have auniform thickness.
 21. (canceled)
 22. The electronic device according toclaim 12, wherein each cell in one EBG layer overlaps an integer numberof cells in another EBG layer.
 23. A method for suppressingelectromagnetic noise in an electronic device having a first conductiveplane and a second conductive plane, comprising: forming at least twoEBG layers having different stopbands between the first conductive planeand the second conductive plane in parallel; and connecting the at leasttwo EBG layers respectively to one of the first and second conductiveplanes through a plurality of vias; wherein at least the vias connectingone of the EBG layers pass through via holes in cells of another EBGlayer.
 24. The method according to claim 23, wherein the at least twoEBG layers are formed such that the stopbands are contiguous.
 25. Theelectronic device according to claim 12, wherein a ratio of a cell sizein one EBG layer to a cell size in another EBG layer is equal to orgreater than 2:1.